1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and in particular, to an exposure mask for a lithography process using a reflective optical system.
2. Description of the Related Art
In recent years, miniaturization of semiconductor devices has increasingly reduced the line width of circuit patterns. In response to a demand for such a reduction, lithography techniques have reduced the wavelength of exposure light used to expose a resist. For a generation with a pattern width of about 45 nm, the use of light called extreme ultra violet (EUV) light in a wavelength region of about 13.5 has been examined. EUV light is expected to enable a reduction in pattern width, pattern size, or pattern pitch, which has not been achieved yet.
However, exposure using EUV light requires a reflective exposure mask and a reflective optical system instead of a light transmissive exposure mask and a light transmissive optical system, which are conventionally used. This is because of the conventional lack of a mask material that allows EUV light, having a wavelength of about 13.5 nm, to pass through if the exposure mask is formed to have a desired thickness.
If a reflective mask is used, light reflected by a mask surface needs to be guided into a projection optical system without interfering with light entering the same mask. Thus, the EUV light entering the reflective light is allowed to impinge obliquely on the mask surface at a predetermined angle (offset angle) (offset incidence). In this case, depending on the angle between the EUV light entering the mask and the mask pattern, a transferred image of the mask pattern on a wafer may differ from the mask pattern. For example, the contrast of the pattern may vary depending on whether or not the long sides of the pattern are arranged orthogonally or parallel to the direction of the incident light. This in turn varies the transferred image of the mask pattern on the wafer. In particular, when the incident EUV light is orthogonal to the mask pattern, the position of the transferred image of the mask pattern on the wafer may move (shift) in the direction of the incident light.
This pattern image shift phenomenon can be corrected by offsetting the entire mask pattern in a direction in which the shift is cancelled or setting an offset value via an exposure apparatus during exposure, as disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2005-72309. However, even with the uniform offsetting of the entire mask pattern, it may be difficult to appropriately correct the shift phenomenon for the entire mask pattern.